The terms polysilazanes, organopolysilazanes, silazane polymers or carbosilazane resins, as they are encountered in the literature, are generally employed to denote solid or more or less viscous liquid polymeric substances containing a plurality of --NR--Si-- units. The term polysiloxazane is preferably employed when the macromolecular chain also contains oxygen. Such polymeric substances may be shaped and converted by pyrolysis into articles which are usually referred to as ceramics, containing silicon, nitrogen and carbon and, if appropriate, oxygen. In most cases, the oxygen is present in the form of silicon carbide, nitrides or carbonitrides.
Various techniques have been proposed in the prior art for preparing the above-mentioned polysilazanes. For example, U.S. Pat. No. 3,853,567 to Verbeek discloses the reaction of a halosilane with an amine. According to the Verbeek reference, the halosilane may be chosen from various silanes which are mono- or polysubstituted with halogen atoms, preferably chlorine, since it is possible for any free silicon valences to be substituted for by alkyl or aryl groups. As for the amines, these may be chosen from compounds bearing one or more NH.sub.2 or NH groups since it is possible for these compounds to be, for example, primary or secondary amines, amides, hydrazines or hydrazides or, in the case of halosilanes containing carbon, ammonia. It should be noted that, in the Verbeek reference, the organic bases preferably employed do not contain an oxygen atom. Most preferable are the compounds consisting solely of nitrogen and hydrogen, such as ammonia. Moreover, the use of anhydrous ammonia is also recommended in U.S. Pat. No. 4,482,669.
The above-described reaction of a halosilane with a compound consisting solely of nitrogen and hydrogen and optionally, carbon, has been carried out, according to the literature in this field, in both the vapor phase and in solvent mediums (see, e.g., U.S. Pat. Nos. 4,397,828, col. 1, lines 23-26 and lines 49-53; 2,564,674 and 3,853,567, which was referred to above).
Moreover, it is known that a problem which occurs during the preparation of polysilazanes concerns controlling the viscosity, the molecular mass and the structure of these polymers. The technique proposed by Verbeek, for example, does not always make it possible to ensure that these factors are adequately controlled (see, also, U.S. Pat. No. 4,310,651). French Pat. nos. 2,584,080 and 2,584,079 and U.S. Pat. No. 4,482,669 propose catalytic or thermal treatments of chlorosilane ammonolysates, so as to increase the viscosity of the silazane and also the yield of the inorganic product obtained when the silazane is pyrolysed.
Additional references describe the syntheses of a ceramic substance containing a major proportion of silicon oxynitride formed by the pyrolysis of an organometallic precursor. In particular, European Pat. no. 167,230 discloses how an optimum content of oxygen in the ceramic makes it possible to inhibit crystallization therein and hence to improve the mechanical properties of this material at high temperatures (i.e., 1200.degree. C. and above). Furthermore, European Pat. nos. 161,828, 181,208 and 162,596 also illustrate the advantages of partially oxidized ceramics.
Another advantage of such partially oxidized ceramics is that they possess an improved resistance to oxidation by air at high temperature. In the patents described above, the introduction of oxygen into the precursor is performed by adding an oxide powder, for example silica, or by the action of moist air on the precursor once it has been shaped. This latter treatment is intended to produce a crosslinking upon the surface of the article so as to endow it with stiffness in the desired form before pyrolysis.
While the siloxazane group has been known for a long time, these are chiefly products produced by the ammonolysis and hydrolysis of chlorosilanes. Recently, it has been proposed, i.e., by Yuan-Fu Yu and Tai Il Mahin Mat. Res. Soc. Sym. Proc. vol. 73, 1986, pps. 559-65 to use a siloxazane obtained by catalytic "hybridization" of a mixture of siloxanes and of silazanes as precursors of a partially oxidized silicious ceramic, for example Si.sub.2 N.sub.2 O.
Furthermore, Wannagat, in Z. Anorg. Allg. Chem. 1971, 385(3), 261-70, has described the synthesis of molecules containing both --Si--O-- and --Si--N--N-- chains. However, these molecules were produced by the coreaction of mixtures of ClMe.sub.2 SiNMeSiMe.sub.2 Cl and of ClMe.sub.2 SiOSiMe.sub.2 Cl with hydrazine or methylhydrazine. These molecules, of low molecular weight (i.e., &lt;350) have, furthermore, never been proposed as ceramic precursors.